Numerous methods of treating wastewater are known. The two prevalent methods include the use of suspended growth systems (SGS) and fixed-film systems (FFS). One type of SGS is the sequencing batch reactors (SBR), which are used for the treatment of wastewater such as sewage or output from anaerobic digesters or mechanical biological treatment facilities in batches. Oxygen is bubbled through the wastewater to reduce biochemical oxygen demand (BOD) and Chemical oxygen demand (COD) to make the wastewater suitable for discharge into sewers or for use on land.
While there are several configurations of SBRs the basic process is similar.
Generally, the SBR comprises at least two identically equipped tanks with a common inlet, which is used to alternate the flow of wastewater between them. The tanks have a “flow through” system. That is, with substantially untreated wastewater (influent) entering the tank at one end, and treated water (effluent) leaving the tank at the other. While one tank is in a substantially inactive (settle/decant) mode, the other tank is actively aerating and filling. At the inlet is a section of the tank known as the bio-selector. This consists of a series of walls or baffles which direct the flow either from side to side of the tank or under and over consecutive baffles. This helps to mix the incoming influent and the returned activated sludge, beginning the biological digestion process before the mixture enters the main part of the tank.
There are four stages to treatment. These stages include fill, aeration, settling and decanting. The aeration stage involves adding air, usually ambient, to the mixed solids and liquid either by the use of fixed or floating mechanical pumps or by blowing it into finely perforated membranes fixed to the floor of the tank. During this period the inlet valve to the tank is open and a returned activated sludge pump takes mixed liquid and solids from the outlet end of the tank to the inlet. This “seeds” the incoming sewage with live bacteria.
Aeration times vary according to the plant size and the composition/quantity of the incoming mixture. The addition of oxygen to the mixture encourages the multiplication of aerobic bacteria, which consume the nutrients. This process encourages the production of nitrogen compounds as the bacteria increase their number, a process known as nitrification.
In the settling stage, the sludge formed by the bacteria is allowed to settle to the bottom of the tank. The aerobic bacteria continue to multiply until the dissolved oxygen is exhausted. Conditions in the tank, especially near the bottom are now more suitable for the anaerobic bacteria to flourish. Many of these, and some of the bacteria which would prefer an oxygen environment, use nitrogen as a base element and extract it from the compounds in the liquid. This process, know as denitrification, consumes the nitrogen compounds created in the aeration stage.
As the bacteria multiply and die, the sludge within the tank increases over time. A waste activated sludge pump removes some of the sludge during the settle stage to a digester for further treatment. The quantity or “age” of sludge within the tank is closely monitored, as this can have a marked effect on the treatment process. The sludge is usually allowed to settle until clear water is on the top 20%-30% of the tank contents.
The decanting stage most commonly involves the slow lowering of a scoop or “trough” into the basin. This has a piped connection to a lagoon where the final effluent is stored for disposal to a wetland, tree growing lot, ocean outfall, or to be further treated for use on parks, golf courses etc.
Fixed-film systems (FFS) are biological treatment processes that employ a medium such as rock, plastic, wood, or other natural or synthetic solid material to a support biomass on its surface and within its, preferably, porous structure. At least two types of fixed-film systems may be considered those in which the medium is held in place and is stationary relative to fluid flow (trickling filter) and those in which the medium is in motion relative to the wastewater (e.g., rotating biological disk). A third classification includes dual-process systems that encompass both fixed and suspended biomass together or in series.
Trickling filter systems are typically constructed as beds of media through which wastewater flows. Oxygen is normally provided by natural or forced ventilation. Flow distributors or sprayers distribute the wastewater evenly onto the surface of the medium. As the wastewater moves by gravity through the medium, soluble and colloidal organic matter is metabolized by the biofilm that forms on the medium. Excess biomass sloughs from the medium and is carried with the treated wastewater to the clarifier, where the solids settle and separate from the treated effluent. At this point the treated wastewater may be discharged or recycled back to the filter medium for further treatment.
A fixed-film biological treatment process that employs rotating disks that move within the wastewater is referred to as a rotating biological contactor (RBC). Developed in the late 1960s, the RBC employs a plastic medium configured as disks and mounted on a horizontal shaft. The shafts are rotated slowly (1 to 2 rpm) by mechanical or compressed air drive. For a typical aerobic RBC, approximately 40 percent of the medium is immersed in the wastewater. Anoxic or anaerobic RBCs (far less common) are fully immersed in the wastewater. Wastewater flows through the medium by simple displacement and gravity. Biomass continuously sloughs from the disks, and some suspended biomass develops within the wastewater channels through which the disks rotate, making the addition of a secondary clarifier necessary. The rotation of the disks exposes the attached biomass to atmospheric air and wastewater. Oxygen is supplied by natural surface transfer to the biomass. Some oxygenation of the wastewater is also created by turbulence at the disk-water interface. The use of exposed and submerged stages in multiple tanks to create aerobic and anoxic conditions may be employed where nitrogen removal is required.
Commercially available modifications primarily address the media employed, the configuration of the tank, and the mechanical supporting systems (e.g., supplemental aeration, programmable cycling, etc.). Some FFS sludges are wasted directly by pumping of the clarifier, whereas others convey all excess solids back to the pretreatment stage (septic tank) for subsequent removal. Lightweight synthetic media have greater surface area and are easier to install. Numerous variations ranging from extruded foam to high-specific-surface PVC and other plastic shapes are available commercially.